An ink-jet recording apparatus employed as an image recording apparatus (an imaging apparatus) such as a printer, a facsimile machine, a copier, or a plotter includes an ink-jet head as a liquid droplet ejecting head including nozzles for ejecting ink droplets, liquid chambers (also referred to as ink channels, ejection chambers, pressure chambers, pressure liquid chambers, or channels) communicating with the nozzles, and driving parts (pressure generating parts) for pressurizing ink in the liquid chambers. Liquid droplet ejecting heads include a head ejecting droplets of a liquid resist and a head ejecting droplets of a DNA sample in addition to an ink-jet head, but the following description is given based mainly on the ink-jet head as the liquid droplet ejecting head.
As an ink-jet head, a so-called piezoelectric ink-jet head is well known. The piezoelectric ink-jet head employs a piezoelectric body, particularly a layered piezoelectric body of alternate layers of piezoelectric layers and internal electrodes, as a pressure generation part generating pressure for pressurizing ink in a liquid chamber. An elastically deformable diaphragm forming a wall face of the liquid chamber is deformed by the displacement of the layered piezoelectric element in the d33 direction, so that the volume/pressure inside the liquid chamber is changed, thereby ejecting ink droplets.
Japanese Laid-Open Patent Application No. 10-286951 discloses an ink-jet head using such a layered piezoelectric element. In this ink-jet head, grooves are formed in part of a layered piezoelectric element having an external electrode formed as individual electrodes on one side and a common external electrode formed on the other side, so that a plurality of driving parts (driving channels) are formed between non-driving parts formed on both ends. The common electrode of the layered piezoelectric element extends from the non-driving parts on both ends in the directions in which the driving parts are arranged.
In recent years, the ink-jet recording apparatus has been required to perform image recording with higher quality at higher speeds. In order to increase recording speed, the number of nozzles of a head is increased in the secondary scanning direction so that a single scan of a carriage in the primary scanning direction has a larger width in the secondary scanning direction in printing.
However, in the case of extending the common electrode of the driving parts of the layered piezoelectric element from the non-driving parts on both longitudinal ends thereof as in the above-described ink-jet head, when the layered piezoelectric element is elongated so as to increase the number of driving parts (driving channels) for the purpose of increasing recording speed, the length of conduction from the common electrode extension parts (non-driving parts) to each of the driving channels increases as the number of driving channels increases. As a result, the resistance of the common electrode increases. Particularly, in the case of forming a plurality of driving parts by performing half-cutting on the layered piezoelectric element, the ungrooved remaining part serves as the common external electrode, so that the long narrow part increases the resistance of the common electrode.
The time constant of a driving voltage applied to a driving channel in the case of driving all the driving channels is different from that in the case of driving only one of the driving channels.
The time constant of the driving voltage increases as the number of driving channels increases. Further, the degree of such increase becomes greater as the resistance of the common electrode increases.
When the time constant of the driving voltage thus changes, an ink droplet ejection characteristic, particularly ink droplet velocity, changes to vary the point of impact of an ink droplet. Therefore, when there is a large difference between the time constants of the foregoing two cases, great degradation is caused in image quality especially in the case of printing a high-density image of approximately 600 dpi.
Therefore, in the conventional ink-jet head using a layered piezoelectric element, there is the problem that high image quality cannot be obtained due to the large resistance of the common electrode especially in the case of printing a high-density image of approximately 600 dpi.